You must be connected to the internet to receive a payment request prompt from the Abra app. Without Internet, the prompt will not be triggered.
When you receive the payment request prompt, you will have the option to approve the payment request if you have sufficient funds in your Abra wallet. Simply press Approve to complete the payment.
If you do not have enough funds to approve the payment request, but do not wish to reject the request, you can also exit the app and await incoming funds (either through depositing via a bank, or by receiving funds from another individual).
After enough funds arrive and are made available to your Abra wallet, you can re-open the app to retrieve the original payment request prompt, and you will be able to approve the transaction.